This invention pertains generally to manufacturing of semiconductor and/or optoelectronic devices and, more particularly, to a method and apparatus for improving uniformity of etching or deposition of a thin film on a substrate in manufacturing of a semiconductor device.
In manufacturing semiconductor integrated circuit and optoelectronic devices, there are a number processing steps where layers of material are patterned or deposited on the substrate. The rate of etching or deposition of such material is often critical to the success of the process and the proper function of the transistors and interconnects in the integrated circuit or to integrated optical components. To guarantee high die yields, these rates must be tightly controlled and uniform across the entire wafer area. Often, such etching or deposition is done in a reactor where the plasma is generated by an inductively coupled source.
Uniformity of etching, deposition rate or deposited film properties on each wafer in such a reactor depends on maintaining good uniformity of the flux with its reactive constituents, ions and radicals to the wafer. This requires a specific profile of power deposition into the plasma from the induction antenna. This profile, depending on the reactor shape and gas pressure, then makes the generation rates of ions and neutral reactive species nearly constant above the wafer. Both these rates are functions of the gas density and electron energy distribution and they must be reasonably uniform spatially. The energy of plasma electrons in inductively coupled plasma sources is provided by the radio frequency electric field that is generated by the excitation coil or coils. Such coil(s) often provide E-M energy to the plasma in the source in a non-axisymmetric way or distributed radially so that there are azimuthal or radially dependent (respectively) non-uniformities in the plasma adjacent to the wafer. Both non-uniformities need to be eliminated to make the plasma properties and the etching or CVD deposition rate uniform.
Inductive coupling of RF power to the plasma in the source is typically done using an RF coil wound approximately helically around an axis-symmetric vacuum vessel. When powered by an RF power source, this coil produces both RF magnetic and electric fields in the source volume—if there is no electrostatic shield. While the inductive electric field (produced by the changing magnetic flux) is efficient in providing energy to electrons and maintaining the plasma, the electrostatic electric fields (arising from the RF potential on the coil) are not necessary and can cause plasma potential modulation. This electrostatic electric field causes poorly controlled sheath potentials, electrical charge damage to the semiconductor and optical devices, and contributes to the sputtering of vessel material onto the substrate. In order to reduce these problems, a slotted electrostatic shield may be placed between the RF coil and the vacuum vessel (see, for example, U.S. Pat. No. 5,534,231, issued to Savas and which is incorporated herein by reference). Such a shield can significantly reduce the undesirable electrostatic fields from conducting displacement currents from the coil into the plasma—which then causes plasma potential modulation and the other aforementioned undesirable effects. An electrostatic shield of any type may be used with an inductively coupled plasma source, but for RF power to penetrate the shield, a slotted electrostatic shield has been found to be an effective approach.
Electrostatic shields, despite their known benefits for process control and avoidance of metal contamination, have up to this time been seen to have little potential for control of the uniformity of the plasma in inductively coupled plasma sources. Applicants are unaware of any attempt thus far to use electrostatic shields to control the uniformity of a plasma or a process using an inductively coupled plasma.
Referring to FIG. 1, a prior art frustoconical-shaped electrostatic shield 101 is diagrammatically illustrated having a plasma generation coil 102 wound about the shield and having evenly spaced slots 103 distributed about its circumference. Inside shield 101 is a plasma containing vessel 104. Slots 103 extend both above and below the coil turns so as to make magnetic field penetration more efficient. Such shields have virtually always been symmetrical with regularly spaced slots of constant width. Accordingly, such a shield configuration has little or no effect on plasma non-uniformity, either azimuthal or radial. Normally, the most persistent non-uniformity in such plasma sources is the radial non-uniformity. Until this time, the normal way to reduce this type of plasma non-uniformity to low levels has been to use a large plasma source diameter. In order to achieve the few percent uniformity required in etching or CVD systems for semiconductors, plasma sources are normally almost twice the diameter of the wafers processed. Commonly, the plasma source is 14 inches to 16 inches in diameter to provide adequate uniformity for 8 inch wafers, whether shielded or not. Unfortunately, such large sources tend to require proportionally larger wafer transport chambers which makes the etching/CVD system more expensive and requiring proportionally more of the very expensive floor space in a semiconductor fabrication facility (Fab). This appears to be the situation to date, irrespective of the fact that there is an economic reason to make the plasma source of the Etch/CVD chamber as small as possible, consistent with the requisite process uniformity.
Smaller size sources with good radial uniformity have been produced, but only so long as the pressure and power ranges of operation are small. Since the shape of the source has a strong influence on the radial variation in the plasma density, it can be made specifically to optimize uniformity for some narrow source conditions. However, the source cannot then be used for processes with substantially different gas pressures or power levels.
This result obtains since the gas pressure strongly affects the transport of energetic electrons in the source and therefore the ionization rate profile. The ability of a plasma source to be useful at widely different pressures and gas compositions is very valuable for process versatility in the IC Fab. Yet, the economic benefits of the smaller sources are substantial and, therefore, it would be desirable to find a way to make such small sources flexible in the conditions for which uniform plasma density can be achieved.
The azimuthal non-uniformity in the plasma density of a source can be significant for high RF frequency for plasma generation or non-helical excitation coil configurations. In the case of high RF frequency this is due to the variation of the RF current as a function of the position on the coil. Frequencies of 13.56 MHz for a 14-inch plasma source typically result in variations of about 10% to more than 20% variation in the magnitude of the RF current on the excitation coil. Therefore, the azimuthal non-uniformity can be substantial and this will cause there to be an azimuthal variation in the power delivered to the plasma. Such azimuthal variation in power injection will cause a similar type non-uniformity of the plasma density, whose magnitude will depend on the pressure of the gas in the source and its size. Yet, use of such a high frequency for powering the source offers benefits since the generators and matching networks are well understood and it is an ISM standard frequency. Therefore, it would be desirable to have an effective way to compensate for the non-axisymmetry of the power injection from a multi-turn excitation coil.
It is submitted that, in the prior art, the slot density or slot size in electrostatic shields has always been axisymmetric and unable to mitigate or reduce the azimuthal non-uniformity due to the asymmetry of power injection into inductively coupled plasma. For example, U.S. Pat. No. 5,234,529, issued to Johnson, uses axially varying slot lengths or locations to adjust the axial location of a plasma formed in a cylindrical source, but assumes axisymmetry, as well as a proper (uniform) radial density of the plasma. In Johnson, the variable length slots, produced by a two-part shield, are used only with a cylindrical shield, that is disposed directly between the RF coil and the plasma chamber. Johnson explicitly teaches that this shield variability may be used for the purpose of adjusting the position of the plasma above the wafer. Specifically, Johnson teaches using the slot shape adjustment only for adjusting the location of the plasma and not at all the shape of the plasma.
Accordingly, there remains an unfulfilled need in the prior art to reduce or eliminate plasma azimuthal non-uniformity for purposes of enhancing the value of high frequency or non-axisymmetric inductively coupled sources in manufacturing processes. Moreover, it would be desirable to provide for uniform, adjustable radial plasma parameters.